Disgaea 4: A Promise Unforgotten
Mana is really important now, since besides passing bills it's also used to learn and boost new skills and pick up Evilities. As soon as you can handle the Item World consistently do try to get a Manager up to 300 ASAP.
You should level monsters, especially if that monster is Desco, because Desco is borked. She's too strong. The game can't handle her. And you should work on a Nekomata or a Phantom Beast because they magichnage into fists, and you need a fused magichange fist for Big Bang to one-shot Mt. Ordeals 4. If you're going to shoot for the highest power level, anyway. A lot of monsters are good actually. Pick ones out you like and work with 'em. Big props to the Alraunes for having Braveheart and other buff spells.
None of the classes in this game are abjectly bad in the way Disgaea 1 had bad classes.
Here's the Evil Symbol guide since some of those unlock conditions are obscure: http://www.gamefaqs.com/vita/723483-disgaea-4-a-promise-revisited/faqs/62958 . The leader of a symbol gets stat boosts from the members so you can prop someone up by having cool dudes hang out. The training ground and aura pyramid are indescribably useful for grinding up units without actually having those wimps fight. The Discipline Room is mandatory for getting to the super-postgame zone Land of Carnage and for duplicating items via Puppy Paw Stick.
Also noteworthy is getting the Star Mage/Skull requires the fire/ice/wind duders at a sum level of 50. Star magic is still ridiculously useful in this game. Thieves are important for stealing since you can't buy things past rank 30 and only find them in enemy inventories and bonus lists. Create Box/Barrel can be used to grind up bonus ranks as long as your Thief's SP holds out and the enemies don't all die.
3-2, 5-2, 9-4, and Ordeals 4 are the big grinding spots, especially if you manage stronger enemy bills correctly. For weird math reasons EXP rewards drop past level 100 and only pick back up at 321. 5-2 and 9-4 change layouts after the first clear to be grinding spots, btw.
Generics double the base reincarnation mana cost each time, capping at 999,999 per. Story characters stay at 100 and gain 5% to their aptitudes for the first five reincarnations.
Always take mystery rooms in the item world because you can find weird and interesting things. Never talk to anyone in a mystery room more than once unless you want to fight.